This invention relates generally to methods and apparatus for computed tomographic (CT) image reconstruction, and more particularly to methods for view weighting of computed tomographic image data.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Multislice helical view weighting is a reconstruction process that generates equivalent axial scans using linear interpolation of helical scan data. Its accuracy and performance affect the overall image quality and reconstruction subsystem performance. In one known system, a z-smoothing algorithm (ZS) has been employed to provide helical view weighting. Although this known algorithm performs reasonably well in four-slice CT imaging systems, the z-smoothing algorithm is not sufficient for eight slice systems because it is too slow.
Slice sensitivity profile is a measurement that is an important indicator of image quality. The data range of the ZS algorithm is limited within 2xcfx80 if overscan correction is not considered. To obtain a desirable signal to noise ratio, more data along the z-direction must be used, which results in a poor slice sensitivity profile.
Helical weighting functions play an important role in the improvement of image quality. The ZS algorithm allows limited variations of weighting functions, and therefore provides less opportunity to further improve image quality. In addition, in known high quality (HQ) Fast/premium modes, the acquired data are more than 2xcfx80 within the image reconstruction range. The ZS algorithm does not fully utilize the data to further improve image quality.
The ZS algorithm also requires a large code size and thus requires a large internal memory.
It would therefore be desirable to provide methods and apparatus for fast multislice helical weighting and smoothing for CT imaging systems designed for imaging more than four slices at a time. It would also be desirable to provide methods and apparatus for helical weighting and smoothing that provides an improved slice sensitivity profile. In addition, it would be desirable to provide more variations of weighting functions and greater opportunity to improve image quality, and to fully utilize data within an image reconstruction range. Furthermore, it would be desirable to provide methods and apparatus for helical weighting and smoothing that does not require a large internal memory.
One embodiment of the present invention is a method for reconstructing an image of an object utilizing a computed tomographic (CT) imaging system. The method includes steps of: helically scanning an object; interpolating an axial fan beam set of projection data as a vector function {right arrow over (R)}a from a fan beam set of projection data from the helical scan {right arrow over (R)}hi, where i=1, . . . , n is a row index and n represents a of number of rows of the detector array, using a relationship written as:                               R          →                a            ⁢              (                  β          ,          γ                )              =                  ∑                  i          =          1                n            ⁢                                    w            i                    ⁢                      (            β            )                          ⁢                                            R              →                                      h              i                                ⁢                      (                          β              ,              γ                        )                                ,
where wi(xcex2) is a weighting function written as:             w      i        =                  ∑                  j          =          1                m            ⁢              f        ⁢                  xe2x80x83                ⁢                  (                      β            -                          β              j                                )                      ,
where m is a number of images used for z smoothing, xcex2j is a gantry rotation angle for a plane of reconstrution of a jth image, and       f    ⁢          xe2x80x83        ⁢          (      x      )        =      {                                                      g              ⁢                              xe2x80x83                            ⁢                              (                x                )                                      ,                                                              "LeftBracketingBar"              x              "RightBracketingBar"                        ≤                          β              b                                                                        0            ,                                                              "LeftBracketingBar"              x              "RightBracketingBar"                         greater than                           β              b                                          
where constants             β      b        =                  2        ⁢                  xe2x80x83                ⁢        π            p        ,
and g(x) is either a linear or non-linear function.
Embodiments of the present invention provide a significant reduction of scan data correction post processing time. Also, embodiments of the present invention provide improved slice sensitivity profile, an ability to further reduce image artifacts and noise through the use of sophisticated weighting functions and all data available in a region, a reduction in memory usage, and a simplification of scan data correction post processing software.